1. Field of the Invention
The present invention relates generally to pressure sensors, and specifically, to fluid media compatible integrated pressure sensors.
2. Background Information
FIG. 1a illustrates a schematic view of a conventional, commercially available semiconductor pressure sensor 1, which is a Motorola.TM. MPX4100 pressure sensor. This device integrates a silicon sensor, bipolar operational amplifier circuitry, and thin film resistor networks to provide an output signal and temperature compensation on a single die. As shown, the pressure sensor 1 includes a sensing element 2, a thin film temperature compensation and first gain stage 3, and a second gain stage and ground reference shift circuitry 4. A vacuum is sealed behind the sensor diaphragm, providing a pressure reference.
FIG. 1b illustrates a cross-sectional view of the conventional pressure sensor 1 of FIG. 1a. Referring to FIG. 1B, the pressure sensor 1 includes a die 5 having a diaphragm 6 and is attached to a backing wafer 7 to form a sealed vacuum reference cavity 8. The backing wafer 7 is attached to an epoxy plastic case 9 by way of a die bond, as shown by numeral 10. The die 5 is interconnected to a leadframe 11 by way of one or more wire bonds 12. A metal or plastic cap 13 having an opening 14 is mounted to the case 9, for directing external pressure to the sensor. The case 9 has a second opening 15 on the opposite side of the opening 14 with an associated hole through the backing wafer 7 for gage or differential pressure sensing. A fluoro or silicone gel 16 is coated over the die 5 to provide a minimal amount of protection for the die surface and wire bonds 12 from harsh environments, while allowing the pressure signal to be transmitted to the diaphragm 6.
However, the pressure sensor 1 has a primary drawback in that its operating characteristics are based on use of dry air as the pressure media. Media other than dry air may have adverse effects on sensor performance and long-term stability.
FIG. 2 is a typical prior art sensor 20 for fluid media compatible applications. The sensor package 20 includes a sensor die 21 which is placed in a metal housing 22 with hermetic glass feed-through pins 23. The sensor die 21 has an integral glass or silicon constraint bottom 24 which provides a sealing cavity 25 therebetween for vacuum reference and stress isolation from the housing. The die attach material is typically a soft material such as RTV. A metal diaphragm 26 is welded to the metal housing 22 and an inside cavity 27 formed therein is filled with oil to allow transfer of pressure to the sensor die 21. This configuration isolates the sensor die 21 from the fluid media. The use of the metal diaphragm is the primary packaging technique available today for more demanding applications, with variations in pressure and ability to apply most fluids to the sensor package. However, this type of package only generally addresses specific applications and is very expensive.
Therefore, there is a need in the industry for a pressure sensor that is compatible with fluid media as well as gaseous media and overcomes the drawbacks mentioned above.